take a bowl, pour in some water, and leave it out in the moonlight. that's the basic way to make it.
myself, i get a cup of water, put it in a bowl, and leave it in the moonlight for one month, make sure you cover it before any sunlight touches it, or it will start to become sun water. after the month, bottle it and keep it in a cool dry place. if sunlight touches it then it will be fine, unless you leave it out in the sun for a couple days.